The frame of a road vehicle normally comprises a central section, which is stiff (i.e. does not collapse in case of crash) and defines a safety cell, inside of which a passenger compartment is obtained, and two end sections (front and rear, respectively) which are collapsible, that is, they deform in controlled manner in case of crash. Thereby, the energy of the crash is absorbed as closely as possible by the end section of the frame involved in the crash, thus allowing the central section of the frame (inside of which the passenger compartment is defined that is to protect the occupants of the vehicle) to resist without deformations and thus at the same time reducing the decelerations to which the central section of the frame is subjected.
Normally, in each end section of the frame there are two or more collapsible struts with controlled deformation, which are oriented longitudinally and serve the function of deforming in controlled manner in case of crash; in particular, when they are loaded axially with an adequate force of compression (as happens during a crash of a certain force), the struts collapse (they compress) axially thus progressively reducing the axial dimension thereof.
In order to minimize the decelerations which the central section of the frame is subjected to in case of a crash at low speed (within 30-40 km/h) against a fixed obstacle, it can be desirable to have highly yielding (that is less resistant and therefore which greatly deform also in the presence of relatively small axial loads) collapsible struts with controlled deformation. Instead, in order to absorb all the energy of a crash at high speed (within 60-70 km/h) against a fixed obstacle, there would be a need to have little yielding (i.e. more resistance and therefore that deform in a significant manner only in the presence of increased axial loads) collapsible struts with controlled deformation. Accordingly, the dimensioning of collapsible struts with controlled deformation is necessarily a compromise between the need to protect the occupants of the road vehicle in case of crash at low speed against a fixed obstacle and the need to protect the occupants of the road vehicle in case of crash at high speed against a fixed obstacle. In other words, the dimensioning of collapsible struts with controlled deformation will allow acceptable results to be obtained in both crashes without however achieving optimal results in either of the two crashes.
Patent Application WO0030916A1, U.S. Pat. No. 6,588,830B1 and Patent Application EP0900717A1 describe a frame of a road vehicle provided with a collapsible strut with controlled deformation; the strut consists of an internally hollow tubular body and presents a thickness that axially varies from an inner end fixed in a projecting manner to the frame, to an outer end. In particular, in Patent Application WO0030916A1 and in U.S. Pat. No. 6,588,830B1, the thickness of the strut varies in a continuous manner, namely without discontinuity, grows continuously from the outer end to the inner end, and presents an absolute minimum at the outer end and an absolute maximum at the inner end.
Nevertheless, the struts described in the above-mentioned prior documents could present a relatively large mass and hence could penalize the performances of the vehicle they are installed in.